Group III nitride semiconductors have a direct transition-type band-gap in an energy range from visible light to ultraviolet light and exhibit an excellent light emission efficiency, and thus group III nitride semiconductors have been manufactured as semiconductor light emitting elements such as a light emitting diode (LED) and a laser diode (LD) for use in various applications. Even when used in an electronic device, group III nitride semiconductors have a potential to provide better properties than cases where conventional types of group III-V compound semiconductors are used.
Such group III nitride semiconductors are, in general, produced from trimethyl gallium, trimethyl aluminum, and ammonia, as raw materials by a Metal Organic Chemical Vapor Deposition (MOCVD) method. MOCVD method is a method in which a carrier gas containing a vapor of a raw material is conveyed to the surface of a substrate, and the raw material is decomposed on the surface of the heated substrate, to thereby grow a crystal.
In the past, a single crystal wafer of a group III nitride semiconductor has not been commercially available, and it has been usual to produce a group III nitride semiconductor by a method of growing a crystal on a single crystal wafer made of a different kind of material. There is a large lattice mismatch between such a different kind of substrate and a group III nitride semiconductor crystal epitaxially grown thereon. For example, when gallium nitride (GaN) is grown on a sapphire (Al2O3) substrate, there is a lattice mismatch of 16% therebetween, and when gallium nitride is grown on a SiC substrate, there is a lattice mismatch of 6% therebetween. In general, if there is such a large lattice mismatch, there is a problem in that it would be difficult to epitaxially grow the crystal directly on the substrate, and even if it is grown, it would be impossible to obtain a crystal having an excellent crystallinity.
Therefore, when epitaxially growing a group III nitride semiconductor crystal on a single crystal sapphire substrate or a single crystal SiC substrate by a Metal Organic Chemical Vapor Deposition (MOCVD) method, a method has been proposed and generally employed that a layer called a low temperature buffer layer made of aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) is firstly laminated on a substrate, and then a group III nitride semiconductor crystal is epitaxially grown thereon at high temperature (for example, refer to Patent Documents 1 and 2).
Other method has also been proposed that a buffer layer is previously formed on a substrate by a sputtering method, the substrate having this buffer layer formed thereon is placed in an MOCVD reacting furnace, and then a group III nitride semiconductor layer is formed thereon (for example, Patent Documents 3 and 4).
According to light emitting elements comprising the group III nitride semiconductors as described in Patent Documents 1 to 4, employing the above-mentioned structure, the occurrence of lattice mismatch between the substrate and the group III nitride semiconductor crystal can be avoided and therefore semiconductor layers of excellent crystallinity can be formed.
In addition, when a semiconductor substrate (wafer) is formed by laminating an underlayer, for example, by using an MOCVD method or the like, on the intermediate layer that has been formed on the substrate using a conventional method as described in Patent Documents 1 to 4 or the like, the entire semiconductor substrate is exposed to high temperature during the formation of the underlayer. Here, as shown in FIG. 5A and FIG. 5B, because the coefficients of thermal expansion are different between the sapphire constituting the substrate 111, and the group III nitride semiconductor such as GaN, the substrate 111 warps typically in such a way that the substrate edge 111b goes to the opposite side of the laminated surface 111a of the substrate 111, and as a result the entire semiconductor substrate is largely warped. If a light emitting element is produced by laminating a semiconductor layer composed of a group III nitride semiconductor on such a largely warped semiconductor substrate, troubles may occur particularly during the exposure in the photolithographic process, the process to grind the backside of the substrate, or the like. For example, if the substrate is largely warped during the exposure by using a photolithographic method, the distance between the photomask and the resist becomes uneven in the substrate plane, and the dimensions of the photomask and the substrate are mismatched in the substrate plane, and as a result, there is a problem that the mask alignment can not be precisely done in the entire substrate plane. Moreover, for cleaving the substrate to divide it into light emitting element chip units, it is necessary to grind the backside of the substrate to be thin. However, if the substrate is largely warped, there is a problem that the substrate may be broken during the grinding process. Furthermore, if the substrate is largely warped at the time of lamination of a semiconductor layer composed of a group III nitride semiconductor, there is a problem that the crystallinity and therefore the light emission characteristics may be deteriorated, because the temperature distribution becomes uneven in the plane, and thus the film thickness and the composition of each layer may become uneven.
In order to suppress such warping of the semiconductor substrate and the light emitting element, a technique has been proposed in which the amount of warping of the substrate is defined within a predetermined range, and a thin epitaxial layer (group III nitride compound: underlayer) is formed on the substrate via an intermediate layer (for example, refer to Patent Document 5). According to the technique described in Patent Document 5, the amount of warping of the substrate is set within a predetermined range, and the underlayer formed thereon is made thin. As a result, the warping of the wafer including the substrate can be suppressed even if the substrate is exposed to high temperature during the process of forming a semiconductor layer on the underlayer.
However, if the underlayer formed on the substrate is made thin as in Patent Document 5, there is a big problem that the crystallinity of the light emitting layer provided on the semiconductor layer formed thereon is deteriorated, and therefore the light emission output power is also lowered and the emission wavelength becomes uneven.